1. Field of the Invention
This invention relates to the field of audio frequency amplification circuits employing field effect transistors as the preamplifier stage for a microphone, preferably an electret microphone.
2. Description of the Prior Art
The use of field effect transistors in audio frequency amplification circuits, particularly preamplifier circuits for high impedance audio signal sources, is well known. Field effect transistor circuitry has been employed in electret and condenser microphone and hearing aid design since the field effect transistor was first invented. For example, U.S. Pat. Nos. 3,512,100 and 4,151,480 and the article entitled "A Transistorized Amplifier for a Condenser Microphone" by Nakamura, et al, appearing in the Journal of the Audio Engineering Society, Vol. 15, No. 2, April 1967, p. 200, disclose various FET preamplifier circuits which generally provide high input impedance and fairly substantial gain.
The '100 patent describes the use of a biasing resistor or pair of parallel but opposed diodes connected between the gate of the FET transistor and the ground terminal of the circuit and power source which, as explained in the '100 patent, reverse bias the gate of the FET and in effect load the microphone input signal at the gate. The diodes also provide a circuit for bleeding off gate leakage and improve the circuit's transient response by dissipating charge between the gate and source of the FET transistor caused by high amplitude output signals of the microphone.
The '480 patent represents an improvement to the '100 patent circuit by the introduction of a second FET transistor functioning as a load in the source to-ground path of the amplifying FET transistor.
The Nakamura article describes the use of a potential dividing biasing network for providing a bias potential to add a voltage less than the output voltage of the voltage source and reaches certain conclusions about the relative bias and source impedance values.
The transducer that Nakamura describes is a condenser microphone using an external source for the polarizing potential as shown in FIG. 4 of the Nakamura article. The series resistance in the polarizing supply acts to limit current, but it is a source of noise. For an electret condenser microphone, no external supply or resistor is needed for the electret itself. Furthermore, for the FET used by Nakamura, the leakage current was 30 pa. For present day devices, with gate-drain voltages under 15 v, it is possible to have leakage currents of about 1 pa. Therefore, the shot noise is greatly decreased over that in the Nakamura circuit. Consequently, the primary noise of preamplifier stage sudio amplifiers used with a modern electret based consenser microphone is the noise produced by the gate resistor. To decrease the noise introduced by the amplifier, it is necessary to make the gate resistor as large as possible.
It may appear from the Nakamura article that a resistor from the gate capacitively coupled to the source will reduce the noise at the input because of "bootstrapping." This confusion arises because for the RC time constant of the input, the effective input impedance is raised and the corner frequency for rolloff is reduced as if a much higher resistor had been used. However, this circuit arrangement very effectively couples the noise of the actual input resistor between the gate and source through the capacitors shown in FIGS. 2 and 4 of the Nakamura article. For this noise, the FET is actually in the common source, not common drain configuration. For a common source amplifier, the voltage gain is greater than unity. The net effect is that the noise referred to the input is at best unchanged. With poor choices for the elements, it is actually increased.
Consequently, it appears that Nakamura did not intend the use of feedback to prevent overload or clipping; his intent appears to be only to prevent a low frequency rolloff.
There is a further drawback to Nakamura's approach--the need for a large value of coupling capacitor to achieve the necessary feedback. The impedance of the coupling capacitor must be less than the parallel combination of the bias resistors if it is to be effective. Using two 1 megohm resistor down to a frequency of 10 Hertz would require a 0.03 fd capacitor. The intent of the audio amplifier circuit of the present invention is to use it inside a very small lapel microphone which would be frustrated by such a large capacitor.
Improvements in the miniaturization of electret microphones which are capable of faithfully transducing a wide frequency and dynamic range of audio signals into electrical signals have outstripped improvements in the preamplifier amplification stage. It has been found desirable to employ the improved miniaturized electret microphone in conjunction with and mounted on acoustic and electric musical instruments which generate high dynamic acoustic signals. The electret microphone in turn generates electrical input signals to the FET preamplifier stage which must respond to and faithfully reproduce a wide dynamic range without clipping.